1. Field of the Invention
The present invention relates in general to methods and apparatus for data synchronisation in a digital communications network.
2. Related Art
In telephony over a passive optical network (TPON), data from a single head-end or central station (exchange) is "broadcast" as a stream of time division multiplexed (TDM) data frames in the downstream direction, over a branched optical fibre network, to multiple optical network units (ONUs), or terminations, typically in customer premises.
Using TPON as an example, ranging is a known process that enables an ONU to transmit data upstream, for example using a time division multiple access (TDMA) protocol, without it being corrupted by data transmitted by other ONUs. In effect ranging compensates for the difference in transmission times between ONUs and the head-end to prevent data from the ONUs, arising from requests from the head-end, colliding at the head-end.
A system of managing data transport, in particular for TPON, in digital communications networks, including a ranging process, is described in the applicants' European patent EP 318 332 B1. In the system described in this patent, the bandwidth in the upstream and downstream directions is arranged into "multiframes", comprising 80 basic frames (for carrying data and housekeeping information) and two basic frames (the ranging slot) for ranging. European patent specification 0618444A concerns a similar system where however coarse ranging signals from an outstation are transmitted at low magnitudes for the duration of a complete multiframe but sampled at the main station only during a ranging window during which other outstations transmit no data. European patent specification 061443A discusses the use of a quasi-oversampled ranging bit pattern for this purpose.
In this description, where necessary for reasons of clarity, the terms "frame" and "slot" are used interchangeably.
In the ranging process, the round-trip time between the head-end and each ONU is determined and a programmable transmission time delay is added to each ONU so that all ONUs have, effectively, the same round-trip time. Typically, the process needs to be accurate to within one data bit.
Ranging generally takes two forms: coarse ranging and fine ranging. Both forms of ranging use a dedicated ranging slot (in TPON, the ranging slot represents less than 2.5% of the available bandwidth of the system). Coarse ranging takes place typically before "on-line" data communications between the head-end and an ONU can begin. Once coarse ranging is complete, for example to an accuracy of 50 ns for TPON, fine ranging is initiated which brings the accuracy up to within 5 ns (within 1 bit period). Fine ranging then continues to operate during normal, "on-line" communications to compensate for drift in the optical system. In the context of this patent specification, "on-line" encompasses when communications between users of a communications network is possible by, for example, telephony, facsimile, television, etc. "Off-line" communications encompasses, for example, system management communications between components of the communications network, for example coarse ranging. System management communications, such as fine ranging, is also possible during on-line operation, using for example dedicated control channels. However, on-line communications is not possible when, for example, the required ONU or head-end is off-line.
In practice the coarse ranging process typically also involves an initial "levelling" procedure to establish an appropriate optical power level at the ONU. For levelling purposes, in response to a signal from the head-end the ONU transmits one pulse in each of a series of ranging slots at a low optical power and increases the optical power until the head-end receives a pulse within its dynamic range. Only then is the coarse ranging procedure started. The combination of levelling and ranging can result in the ranging slot being used many times by the same ONU to adjust its optical power and obtain a correct delay. For convenience, and unless otherwise stated, references to coarse ranging on start-up should be taken to include levelling.
Using TPON as an example, for a PON having 128 ONUs and implementing 10 ms duration multiframes comprising 82 basic frames of 121.9 .mu.s duration (only two basic frames in each multiframe being allocated to ranging), ranging for all ONUs would take up to 5.12 seconds, assuming four ranging slots to range one ONU. While around 5 seconds might not seem a long time, if it is considered that this time represents a network down-time or "dead zone", it can be appreciated that it is a significant amount of time, after, for example, a physical re-configuration of a PON or a head-end crash and before communications may be re-established.
Recently, the PON concept has been extended by incorporating optical amplification stages into one or more of the optical fibre branches of a PON. SuperPONs can have a far greater reach than a standard PON, and can serve around 3500 ONUs.
Applying the values of the above ranging example to ranging a SuperPON, ranging would take around 140 seconds. However, in reality ranging for a SuperPON would take much longer when account is taken of the greater reach, and hence the greater round trip delays, of the SuperPON. In fact, the applicants have shown that ranging all ONUs in a SuperPON may take as long as an hour, which as a dead zone is totally unacceptable.